This is where I began, my knowledge dates from this point. For example, Do you have anything more to add, because if not, this is where I came in. This idiom, dating from the 1920s, originally alluded to the continuous showing of a motion picture, with customers entering the theater at any stage while the film was running and leaving when it reached the point where they had started.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.